In electrophotographic (EP) printers, such as a laser printer, a photoconductive drum is typically used as the source object from which the image is initially formed by dots of laser light impacting the surface of this drum. The photoconductive drum is typically charged to a substantial voltage, such as a voltage greater than 1,000 VDC. This voltage could be either positive or negative with respect to ground, depending upon the charging system and the chemicals used in the photoconductive drum material. Additionally, an AC voltage superimposed on the DC voltage could be used.
For this photoconductive drum to achieve this substantially large voltage, it is typical for a charge roller to be placed into contact with the surface of the photoconductive drum. The charge roller typically comprises a moderately electrically conductive cylinder, or a semiconductive cylinder, which has an electrically conductive center that receives a high voltage from a high voltage power supply. As voltage is received at the electrically conductive center, this voltage charges the entire charge roller, including its outer cylindrical surface. This high voltage at the cylindrical surface of the charge roller is then passed onto the outer surface of the photoconductive drum as the drum rotates.
In laser printers manufactured by Lexmark International Inc., the charge roller is mounted in the printer, and the photoconductive drum is mounted in a removable and replaceable process cartridge. A felt wiper is provided to clean contamination from the surface of the charge roller, and is renewed with every new process cartridge replacement. The life of a process cartridge is a maximum of approximately 25,000 prints, whereas the life of the charge roller is a minimum of 250,000 prints. It is recommended to replace the charge roller itself at scheduled maintenance intervals of 250,000 prints, since the charging characteristics of the charge roller change over time.
The ability of the charge roller to charge the photoconductive drum decreases over its life due to roller characteristics and contamination of the surface of the roller. This decrease in voltage could, over time, impact the ability of the photoconductive drum to produce accurate prints. Consequently, it would be an improvement to be able to compensate for the changing characteristics of the charge roller over its life span.